This invention relates to a high-frequency heating device capable of variably controlling a high-frequency output continuously or in a stepwise manner.
Where food is cooked using a high-frequency heating device such as an electronic oven etc. it is generally known to use a high-frequency wave of about 2450 MHz. The high-frequency output of the electronic oven of this type has been heretofore controlled by effecting a repeated ON-OFF operation of a power source by means of mechanical contacts. In this control method, however, the range of control is restricted and it is impossible to vary a high-frequency output continuously or in a stepwise manner over a wider range. Furthermore, since the contact is frequently operated in an ON-OFF fashion, damage to the contact is liable to occur with the resultant short life of the contact and, consequently, this method proves less reliable and disadvantageous from the practical viewpoint.
To obviate such drawbacks, use is made, in place of such mechanical contacts, of a semiconductor controlled rectifying element such as a thyristor in an attempt to control the thyristor in an ON-OFF fashion according to a pulse generated from a pulse oscillator and at the same time control the ratio between the ON time and OFF time of the thyristor by varying a pulse width. In this control method, however, since the timing in which the thyristor is switched over from a nonconductive state to a conductive state is varied with the level of a power source voltage a larger surge current is generated from a high-tension transformer for supplying a high voltage to a magnetron each time the thyristor is rendered conductive. The service life of the thyristor is therefore shortened due to such a large surge current and, in the worst case, the thyristor is destroyed with the attendant disadvantage. Furthermore, since the timing in which the thyristor is rendered conductive is varied, a transient magnetic flux occurs each time the thyristor is rendered conductive. This causes an electromagnetic sound (beat) to be generated from an electronic oven to give an unpleasant feeling to a neighboring person.
It is accordingly the object of this invention to provide a high-frequency heating device capable of controlling a high-frequency output continuously or in a stepwise manner without involving any large surge current and any electromagnetic sound.
According to one aspect of this invention there is provided a high-frequency heating device comprising a semiconductor controlled rectifying element, a transformer with a primary winding coupled through the semiconductor controlled rectifying element to an AC power source, a high-frequency oscillator coupled to a secondary winding of the transformer, a signal generating circuit operated in accordance with a cycle of an AC power source voltage to generate a signal at a predetermined phase of each cycle of the power source voltage, an astable multivibrator whose output pulse width is variable, and a control circuit for generating an output signal upon receipt of both an output of the astable multivibrator and output of the signal generating circuit to control the semiconductor controlled rectifying element in an ON-OFF fashion.